1. Field of the Invention
The present invention relates to a NOx-decomposing electrode having certain or high decomposing/reducing ability with respect to NOx so that NOx is decomposed to produce oxygen during this process. The present invention also relates to a NOx concentration-measuring apparatus for measuring NOx contained, for example, in atmospheric air and exhaust gas discharged from vehicles or automobiles.
2. Description of the Related Art
A technique has been hitherto known as the method for measuring NOx in a measurement gas such as combustion gas, in which the NOx-reducing ability of Rh is utilized while using a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia to measure an electromotive force generated between the both electrodes.
However, such a sensor suffers the following problem. That is, the electromotive force is greatly changed depending on the change in concentration of oxygen contained in the combustion gas as a measurement gas. Moreover, the change in electromotive force is small with respect to the change in concentration of NOx. For this reason, the conventional sensor tends to suffer influence of noise.
Further, in order to bring out the NOx-reducing ability, it is indispensable to use a reducing gas such as CO. For this reason, the amount of produced CO is generally smaller than the amount of produced NOx under a lean fuel combustion condition in which a large amount of NOx is produced. Therefore, the conventional sensor has a drawback in that it is impossible to perform the measurement for a combustion gas produced under such a combustion condition.
In order to solve the problems as described above, for example, Japanese Laid-Open Patent Publication No. 8-271476 discloses a NOx sensor comprising pumping electrodes having different NOx-decomposing abilities arranged in a first internal space which communicates with a measurement gas-existing space and in a second internal space which communicates with the first internal space, and a method for measuring the NOx concentration in which the O.sub.2 concentration is adjusted by using a first pumping cell arranged in the first internal space, and NO is decomposed by using a decomposing pumping cell arranged in the second internal space so that the NOx concentration is measured on the basis of a pumping current flowing through the decomposing pump.
Further, Japanese Laid-Open Patent Publication No. 9-113484 discloses a sensor element comprising an auxiliary pumping electrode arranged in a second internal space so that the oxygen concentration in the second internal space is controlled to be constant even when the oxygen concentration is suddenly changed.
A cermet electrode composed of Rh/ZrO.sub.2 is used for the NOx-decomposing electrode of the NOx sensor as described above. When the cermet electrode composed of Rh/ZrO.sub.2 is used for the NOx-decomposing electrode, a phenomenon has been observed, in which the sensitivity is lowered in accordance with the increase in operation time.
Rh tends to be oxidized in a range of 500 to 1000.degree. C., and it repeats the oxidation reaction and the reduction reaction depending on the oxygen concentration in the atmosphere. The NOx-decomposing electrode is peeled off from the solid electrolyte substrate due to the change in volume of Rh caused by the repetition of the oxidation reaction and the reduction reaction. As a result, the impedance of the pumping cell is increased during the use of the gas sensor, and the increase in impedance consequently causes the decrease in sensitivity to NOx.